Comment on “Charge Transfer-Triggered Bi3+ Near-Infrared Emission in Y2Ti2O7 for Dual-Mode Temperature Sensing”

Undoped Y2Ti2O7 exhibits impurity emission bands at low temperatures due to Mn4+ and Cr3+, as established by codoping with these ions. Contrary to a recent report by Wang et al., ACS Appl. Mater. Interfaces2022, 14, 36834–36844, we do not observe Bi3+ emission in this codoped host, as also is the case for Fe3+. The emission reported in that paper as being due to Bi3+ in fact corresponds to Cr3+ emission. The Cr3+ and Mn4+ emissions are quenched with increasing temperature, so that Mn4+ emission is scarcely observed above 80 K. We present variable temperature optical data for Y2Ti2O7 and this host codoped with Mn, Cr, Fe, and Bi, as well as a theoretical justification of our results.


INTRODUCTION
Bismuth luminescence has recently attracted much attention, both theoretically 1−6 and experimentally, 7−12 because of potential applications as phosphors for light-emitting diodes, quantum-cutting materials suitable for silicon solar cells, and ratiometric temperature sensors. 13It is an attractive and versatile phosphor because its emission can occur in the ultraviolet (UV), green, red, near-infrared, and infrared spectral regions, depending upon its oxidation state 14 (Figure 1).
The emission of Bi 3+ has been reported in many oxide systems, including oxides, 5,15 aluminates, 2,16,17 zirconates, 18 germanates, 19,20 silicates, 21 scandates, 22 vanadates, 23 titanates, 24 and phosphates, 25,26 and it is a coactor in many of these systems.Historically, Bi 3+ has been known as a near UVemitter, from 3 P 1 , 3 P 0 states to the 1 S 0 ground state, usually in the region of 300−400 nm, although the host lattice can modify the emission wavelength. 27The Stokes shift of Bi 3+ is relatively small: ∼30 nm in Cs 2 NaBiCl 6 , 14 ∼60 nm in Bi 3+doped LiScGeO 4 , 28 and 38 nm for Ca 10 P 6 O 25 doped with Bi 3+ . 29Blasse and Bril 30 also recognized that Bi 3+ could be a universal sensitizer.Unlike Bi 2+ , it is not toxic, and systems doped with it are more easily synthesized.A Scopus search on May 2, 2023, showed that only 166 documents published between 1966 and 2000 concerned the emission of Bi 3+ .However, now the total is 1964 papers, up to 2023, showing the surge of interest in this sensitizer/activator.
We were therefore surprised to encounter a publication concerning room temperature Bi 3+ emission in Y 2 Ti 2 O 7 at 744 nm (lifetime ∼17 μs), with the Stokes shift of 353 nm. 12 The emission band sharpens at 77 K with a maximum at 737 nm.Besides, excitation bands at 485 and 645 nm were reported in addition to the dominant band at 391 nm.These do not correspond to Bi 3+ energy levels.Overall, we did not find the explanation of the emission band as Bi 3+ −Ti 4+ metal−metal charge transfer to be convincing.The broad band emission, due to O 2− (2p) → Ti 4+ (3d 0 ) charge transfer (CT), peaks at 485 nm.The corresponding excitation peak is at 310 nm.These wavelengths are in agreement with those from the earlier study of Alarcon and Blasse. 32The incorporation of Bi 3+ into the Y 3+ site shifts both bands to longer wavelengths, so that the peak of the emission band is located at 550 nm for ∼5% Bi 3+ doping. 31he excitation band was assigned to the Bi 3+ (6s 2 ) → Ti 4+ (3d 0 ) metal−metal CT transition. 31The CT emission band is quenched at low temperature: the intensity decreases by a factor of 8−9 from 10 to 150 K.These phenomena are analogous to the metal−metal CT bands assigned in YVO 4 :Bi 3+33 and La 2 Zr 2 O 7 :Bi 3+ . 34It therefore appears that the previous results for Y 2 Ti 2 O 7 :Bi 3+ call into question the more recent work of Wang et al. 12 We are aware of the effects of minute amounts of impurity ions, notably transition metals, upon host luminescence. 2otably, the luminescence of Cr 3+ -doped Y 2 Ti 2 O 7 has been investigated both experimentally 35 and theoretically. 36,37Also, the optical spectra of Mn 4+ in Y 2 Ti 2 O 7 have been investigated at low temperature. 38,39In addition, Kushnikova et al. 40 reported an intense broad absorption band at 505 nm, attributed to the 5 E g → 5 T 2g transition of Mn 3+ ions replacing Y 3+ .
The aim of the present study is to provide a valid assignment for the previously published luminescence of Y 2 Ti 2 O 7 , attributed to Bi 3+ doping, 12 and we therefore have a useful framework upon which to base our reinvestigation of the spectra of Y 2 Ti 2 O 7 doped with tri-and tetra-positive ions.

RESULTS AND DISCUSSION
2.1.Host Crystal Structure.Crystals and powders of Y 2 Ti 2 O 7 have been grown in previous studies by several methods, including microwave synthesis, 41 the optical floating zone technique, 42,43 and the straightforward solid state solution method, 12 as employed herein (Section 4).The structure of Y 2 Ti 2 O 7 has been described by Wang et al. 12 It crystallizes in space group Fd3m (No. 227), Z = 8, with YO 8 and TiO 6 units interconnected by shared oxygens.Each cation has D 3d site symmetry.A sketch of the structure is made in Figure 2a, and our XRD results are compared with the standard card in Figure 2b.
2.2.Optical Properties of Undoped Material.Figure 3a shows the 355 nm excited emission spectrum between 17 and 327 K of undoped Y 2 Ti 2 O 7 prepared from 99.995% TiO 2 and 99.999% Y 2 O 3 .The spectra recorded using 375 and 395 nm excitation are similar, but when normalizing at 473 nm, the relative intensities of lower energy bands decrease with longer wavelength excitation (not shown).Three groups of bands are observed.The broad band at the shortest wavelength (∼470 nm in Figure 3a) under 355 nm excitation is almost quenched at room temperature.Upon shorter wavelength excitation, Figure 3b, a band is observed at 530 nm, similar to the broad band observed by Alarcon and Blasse under 300 nm excitation for Y 1.998 Eu 0.002 Ti 2 O 7 at 4.2 K. 32 This feature has previously been associated with O 2− −Ti 4+ CT. 31,32 We make preliminary assignments of the ∼470 and 530 nm bands.According to the formation energies, there is only a very small probability of Y− Ti antisites.Hence, the first of these bands is associated with unperturbed (Ti 3+ , O − ) to (Ti 4+ , O 2− ) CT emission and the latter to the scenario of perturbed CT emission involving either Ti 4+ near an oxygen vacancy or O 2− near a cation (Y 3+ ) vacancy.The large breadth, excitation wavelength dependence, and low temperature quenching are therefore not unexpected.
The sharper structure beginning at 661 nm is quenched completely at room temperature.Third, the structured group of bands beginning at 714 nm becomes a broad band at room temperature.The 77 K spectrum in Figure 3a is very similar to our digitized 78 K emission spectrum of Figure 2a in ref 12, to wavelengths longer than 714 nm.The 237 K spectrum is also similar to the 298 K emission spectrum in the same figure.Our photomultiplier and grating setup has a cutoff after 730 nm, whereas the range of the Hamamatsu R928P photomultiplier tube employed in the equipment of ref 12 is from 180 to 920 ) termed metal to metal, Bi 3+ to Bi 3+ intervalent, and ligand to Bi 3+ charge transfer transitions, which involve the transfer of an electron from the former (denoted as an open circle) to the latter (denoted as a filled circle).The emission of Bi 3+ can be 3 P 0,1 to 1 S 0 , CT, or MMCT, depending on which of these three sets of excited states is lower in energy.For Bi 2+ , 2 P 3/2 ′ and 2 P 3/2 ′′ represent the two crystal field levels of 2 P 3/2 .UV−vis, Red, and NIR represent ultraviolet to visible, red, and near-infrared emissions.nm.This accounts for the extension of bands to longer wavelength in the spectra of ref 12.
The corresponding excitation spectra are shown in Figure 3c  and d and are quite similar for the 674 and 736 nm emission bands.The excitation spectrum of 520 nm emission (not shown) has a prominent band at 312 nm.
The lifetime of the 674 nm emission decreases from 0.48 ms at 18 K to 0.28 ms at 58 K.The lifetime of the emission at 737 nm can be followed to more elevated temperatures, and the recorded curves are shown and fitted in Figure 3e to give lifetimes in the millisecond or subms range.Fitting these lifetimes to a single barrier model (Figure 3f) indicates an activation energy in the region of 270−350 cm −1 .Note that the lifetime of the bands at 719 and 737 nm at 288 K of our undoped sample is measured as 13 μs.The corresponding lifetime in Figure 2b of ref 12, attributed to Bi 3+ emission, is 17 μs.
Our undoped sample of Y 2 Ti 2 O 7 therefore contains impurity bands from species present in the starting materials or acquired during synthesis.The comparison with information presented in the Introduction clearly shows that the group of bands at shorter wavelengths of 715 nm corresponds to Mn 4+ emission, whereas bands at longer wavelengths are due to Cr 3+ .We therefore synthesized samples of Y 2 Ti 2 O 7 doped with these materials to confirm our suspicions.

Optical Properties of Mn
-Doped Y 2 Ti 2 O 7 .Figure 4a shows the 18 K spectrum of 1% Mn-doped Y 2 Ti 2 O 7 by using different Xe excitation lines of the Fluorolog instrument.Only one group of bands is observed, between 660 and 700 nm, corresponding to the 2 E ( 2 G) → 4 A 2 ( 4 F) transition.The lifetime is 0.24 ± 0.03 ms, which is shorter than in undoped Y 2 Ti 2 O 7 .We are unable to assign the spectrum under higher resolution (Figure 4b) because the emission is almost quenched at 80 K. Possible reasons for the quenching are transfer to quenching centers or nonradiative relaxation to the electronic ground state.If the ZPL is at 661 nm, one would expect to observe hot bands at 79 K; otherwise, if the ZPL is assigned to the sharper band at ∼670 nm, then the Stokes and anti-Stokes vibrational energies do not match.The presence of several Mn 4+ sites could be implied.with calibration differences between 14 and 33 cm −1 for the lines (denoted by Δ in Table 1).The D 3d double group site symmetry of Cr 3+ spin−orbit levels in CrO 8   or CrO 6 leads to splitting of the excited states.Our calculation shows that Cr is unlikely to occupy the Y site due to high formation energy, even in the case of Y 2 O 3 being depleted.The two sharp bands (lines 1 and 2) are assigned to the Γ 4 , Γ 5,6 ( 2 E) → 4 A 2 zero phonon lines, separated by ∼70 cm −1 .This splitting pattern is repeated upon the structure to lower energy.Becker has made spectral assignments in Table 1 which employ progressions in a mode of 102 cm −1 (E′), together with another vibration of 22 cm −1 (E′′).We do not concur with these assignments except to agree that lines 1 and 2 correspond to ZPL and that all of the remaining structure is vibronic in nature.The sparse data do not merit further discussion.The lifetime of the Cr 3+ emission was measured as 0.25 ms at 17 K.The excitation spectra (Figure 5b) show bands at ∼385 and 480 nm when monitoring the Cr 3+ emission.The Cr 3+ emission is quenched for Cr 3+ concentrations above 3%.As shown by Figure 7a, the main intrinsic defects are V Y ′′′ and oxygen vacancies (V O •• ).The dashed vertical line represents the Fermi level under charge equilibrium under dilute limits of Cr and Mn dopants, and the Fermi level will move downward as the doped defect concentration increases due to charge neutrality constraints.Under dilute limits, the main doped defects are Cr Ti ′ (Cr 3+ occupying Ti 4+ site) and Mn Ti × (Mn 4+ occupying Ti 4+ site), which are followed by Mn Y ′ (Mn 2+ occupying Y 3+ site), and their formation energies will, respectively, increase by 0, −2/3 μ, and +1/3 μ as Δμ O increases by μ.Therefore, the more reducing atmosphere will favor the appearance of Mn Y ′.

CONCLUSIONS
The emission spectrum of our undoped sample of Y 2 Ti 2 O 7 exhibits bands due to Cr 3+ and Mn 4+ impurities.The starting materials were 99.995% TiO 2 and 99.999% Y 2 O 3 .However, the use of an Al 2 O 3 crucible is known to introduce a Cr 3+ impurity.The source of the Mn 4+ impurity is not known.The spectrum of Cr-doped Y 2 Ti 2 O 7 clearly shows that the previously reported Bi 3+ emission 12 in fact corresponds to Cr 3+ .The emission lifetime was reported as 17 μs. 12Our measurement at 288 K gives the value 13 μs.Analysis of the starting materials and the alumina crucible by inductively coupled plasma-mass spectrometry (ICP-MS) shows that the elements Cr, Mn, Fe, and Bi are present at ppb levels (Supporting Information).
The Cr 3+ and Mn 4+ emissions are quenched with an increasing temperature.In fact, the Mn 4+ emission is barely visible above 80 K.We speculate about the possible processes involved.For Cr 3+ , its excited state may ionize to Cr 4+ , and the electron converts Ti 4+ to Ti 3+ .On the other hand, the excited Mn 4+ may ionize to form Mn 3+ and a hole.

EXPERIMENTAL SECTION
x mol % Fe 3+ (x = 0.1, 0.5, 1, and 3), Y 2−x Ti 2 O 7 : x mol % Mn 4+ (x = 0.1 and 3), and Y 2−x Ti 2 O 7 : x mol % Mn 3+ : x = 0.1 and 3).Phosphors were synthesized by a high-temperature solid-state reaction method.Y 2 O 3 , TiO 2 , Fe 2 O 3 , Bi 2 O 3 , MnO 2 , and Cr 2 O 3 were weighed according to the stoichiometry of the chemical formula.Here, 2 mL of ethanol was added to the mixtures, and the mixtures were fully ground in an agate mortar until the ethanol was evaporated.The evenly mixed powder was then transferred to an aluminum crucible, treated at 1000 and 1600 °C for 4 and 10 h, respectively, with intermediate grinding.
After regrinding, the final phosphors were obtained.

Characterization.
The X-ray diffraction (XRD) patterns of samples were collected by a Bruker AXS D8 Advance X-ray diffractometer at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5418Å).The photoluminescence spectra at room and lower temperatures were recorded by a Horiba Fluorolog-3 spectrophotometer, using a 450 W xenon lamp as the light source.A 355 nm Spectra LED was employed to measure the decay curves, with a minimum range of 340 μs.A CS202-DMX-1AL cryostat from Advanced Research Systems (DE-202 series closed-cycle cryostat) was employed to record spectra in the nominal temperature range from 10 to 288 K. X-ray photoelectron spectra were obtained using a Thermo Scientific ESCALAB 250Xi instrument.ICP-MS (inductively coupled plasma−mass spectrometry) analysis was performed using an Agilent Technologies 7900 series ICP-MS instrument.The samples for ICP-MS measurements were prepared by dissolving 1 mg of Y 2 O 3 , TiO 2 , or alumina crucible in 1 mL of concentrated nitric acid (70%, trace metal basis) at 200 °C for 24 h.The resulting solution was diluted with milli-Q water to reach a total volume of 50 mL for measurements.
4.4.Calculations.The calculations were performed with the VASP code 44,45 by employing the projected augmented wave method. 46The Perdew−Burke−Ernzerhof revised for solids (PBEsol) functional 47 was chosen, and the cutoff energy is 520 eV.The single Γ-point sampling was adopted in the supercell calculations.The convergence criteria of energy and residual atomic forces are 10 −5 and 10 −2 eV Å −1 , respectively.For the correction on the localization of d orbitals, the PBE+U method 48 (U = 4 eV) is used in the formation energy calculation, and the Heyd−Scuseria−Ernzerhof hybrid (HSE06) functional 49 with α = 0.25 is used in the charge transition level (CTL) calculation.More details about the calculation of formation energy and charge transition levels of a point defect and the correction have been given in previous work. 2ASSOCIATED CONTENT

Figure 1 .
Figure1.Some of the possible emission transitions of bismuth in undoped or doped compounds.MMCT, IVCT, and CT, respectively, represent Bi 3+ to host cation ion (Ti 4+ in Y 2 Ti 2 O 7 ) termed metal to metal, Bi 3+ to Bi 3+ intervalent, and ligand to Bi 3+ charge transfer transitions, which involve the transfer of an electron from the former (denoted as an open circle) to the latter (denoted as a filled circle).The emission of Bi 3+ can be 3 P 0,1 to

Figure 2 .
Figure 2. (a) Structure of Y 2 Ti 2 O 7 .(b) XRD trace of undoped sample and comparison with the standard card.

Figure 3 .
Figure 3. Variable temperature emission spectra of Y 2 Ti 2 O 7 under (a) 355 nm and (b) 300 nm excitation.Excitation spectra of the (c) 674 nm and (d) 736 nm emission spectra.The sharp features at ∼480 nm here and in subsequent spectra are Xe lamp lines.(e) Decay curves of 737 nm emission using 355 nm excitation, from 18 to 288 K. (f) Single barrier model fit to monoexponential lifetimes determined from panel (e).

2 . 4 .
Optical Properties of Cr-Doped Y 2 Ti 2 O 7 .The emission spectrum of Y 1.98 Cr 0.02 Ti 2 O 7 is shown in Figure 5a.It is very similar to the 77 K spectrum of Cr-doped Y 2 Ti 2 O 7 reported by Becker,

Figure 4 .
Figure 4. (a) Emission spectrum of Y 2 Ti 2 O 7 doped with 0.1% Mn at 18 K using various excitation lines.(b) Temperature dependence of higher resolution spectrum and (c) corresponding excitation spectrum of panel (a).

Figure 5 .
Figure 5. (a) Emission spectrum of Cr-doped Y 2 Ti 2 O 7 at three temperatures as indicated.Refer to Table 1 for the line energies.(b) Excitation spectra of Cr-doped Y 2 Ti 2 O 7 when monitoring different emission bands at 17 K.

Figure 7 .
Figure 7. (a) The formation energies of the intrinsic, Cr-doped, and Mn-doped defects in Y 2 Ti 2 O 7 , where the dashed vertical line represents the Fermi level under charge equilibrium with negligible doping concentration.In the calculation of the formation energies, the atomic chemical potentials are chosen as −2.0 eV (O), −0.3 eV (Ti), −0.3 eV (Y), 0.0 eV (Cr), and 0.0 eV (Mn) by taking the usual phases of O 2 , TiO 2 , Y 2 O 3 , Cr 2 O 3 , and Mn 3 O 4 as references.It is noted the actual formation energies of all the defects of the same dopant (Cr or Mn) can be shifted upward (but not downward) according to the doping concentration, as the Cr-or Mn-rich condition has been employed.(b) The charge transition levels of main Cr-doped and Mn-doped defects, which are calculated with the HSE06 functional.The dashed lines close to the band edge represent the polaron levels of the VBM and CBM.

Table 1 .
Emission Spectra of Y 2 Ti 2 O 7 :Cr 3+a a The energies are in cm −1 , and Δ represents the calibration difference of this work and ref 35.Refer to Figure5afor line numbers.The stars in column 6 represent a typo in the original paper.